Most the prior art manual control boats are controlled through paddles instead of rudders. However in changing the direction of the boat, the paddles are used, but the paddle is operated at only one side of the boat so that the speed of the boat must be reduced and the gravitation center of the boat is displaced.
In another boat structure disclosed in U.S. Pat. No. 6,612,252, a boat has two treadles controlled by legs of the drivers for controlling the direction change of the rudder. Ropes are connected between the treadles and the rudders. In this example, the ropes are soft. They only have the function of changing the direction of rudder. However the water flow under the boat cannot be detected by drivers immediately.